In lithography for device manufacture, there is an ongoing desire to reduce the size of features in a lithographic pattern in order to increase the density of features on a given substrate area. Patterns of smaller features having critical dimensions at nano-scale allow for greater concentrations of device or circuit structures, yielding potential improvements in size reduction and manufacturing costs for electronic and other devices. In projection photolithography, the push for smaller features has resulted in the development of technologies such as immersion lithography and extreme ultraviolet (EUV) lithography.
As an alternative, so-called imprint lithography generally involves the use of a “stamp” (often referred to as an imprint template) to transfer a pattern onto a substrate. An advantage of imprint lithography is that the resolution of the features is not limited by, for example, the emission wavelength of a radiation source or the numerical aperture of a projection system. Instead, the resolution is mainly limited to the pattern density on the imprint template.
For both projection photolithography and for imprint lithography, it is desirable to provide high resolution patterning of surfaces, either of imprint templates or of other substrates. The use of self-assembly of block copolymers (BCPs) has been considered as a potential method for increasing the feature resolution to smaller dimensions than those obtainable by prior art lithography methods or as an alternative to electron beam lithography for preparation of imprint templates.
BCPs comprise different blocks, each comprising identical monomers, and arranged side-by side along the polymer chain. Each block may contain many monomers of its respective type. So, for instance, an A-B block copolymer may have a plurality of type A monomers in the (or each) A block and a plurality of type B monomers in the (or each) B block. An example of a suitable BCP is, for instance, a polymer having covalently linked blocks of polystyrene (PS) monomers (hydrophobic block) and polymethylmethacrylate (PMMA) monomers (hydrophilic block). Other BCPs with blocks of differing hydrophobicity/hydrophilicity may be useful. For instance tri-block copolymers (A-B-C) may also be useful, as may alternating or periodic block copolymers (e.g. [-A-B-A-B-A-B-]n or [-A-B-C-A-B-C]m where n and m are integers). The blocks are connected to each other by covalent links in a linear or branched fashion (e.g. star or branched configuration).
Self-assemblable BCPs are compounds useful in nanofabrication because they may undergo an order-disorder transition on cooling below a certain temperature (order-disorder transition temperature To/d) resulting in phase separation of copolymer blocks of different chemical nature to form ordered, chemically distinct domains with dimensions from tens of nanometers to dimensions which are below 10 nm. The size and shape of the domains may be controlled by manipulating the molecular weight and composition of the different block types of the copolymer. The interfaces between the domains may have line width roughness of the order of 1-5 nm and may be manipulated by modification of the chemical compositions of the blocks of the copolymers.
The feasibility of using thin films of BCPs as self-assembling templates was demonstrated by Chaikin and Register, et al., Science 276, 1401 (1997). Dense arrays of dots and holes with dimensions of 20 nm were transferred from a thin film of poly(styrene-block-isoprene) to silicon nitride substrates.
BCPs may form many different phases upon self-assembly, dependent upon the volume fractions of the blocks, degree of polymerization within each block type (i.e. number of monomers of each respective type within each respective block), the optional use of solvents and surface interactions. When applied in thin films, geometric confinement may pose additional boundary conditions that may limit the phases formed. In general spherical (e.g. cubic), cylindrical (e.g. tetragonal or hexagonal) and lamellar phases (i.e. self-assembled phases with cubic, hexagonal or lamellar space-filling symmetry) are practically observed in thin films of self-assembled BCPs.
The phase type observed may depend upon the relative molecular volume fractions of the different polymer blocks. For instance, a molecular volume ratio of 80:20 will provide a cubic phase of discontinuous spherical domains of the low volume block arranged in a continuous domain of the higher volume block. As the volume ratio reduces to 70:30, a cylindrical phase will be formed with the discontinuous domains being cylinders of the lower volume block. At 50:50 ratio a lamellar phase is formed. With a ratio of 30:70 an inverted cylindrical phase may be formed and at a ratio of 20:80, an inverted cubic phase may be formed.
Suitable BCPs for use as self-assemblable polymers include, but are not limited to, poly(styrene-b-methylmethacrylate), poly(styrene-b-2-vinylpyridone), poly(styrene-b-butadiene), poly(styrene-b-ferrocenyldimethylsilane), poly(styrene-b-ethyleneoxide), poly(ethyleneoxide-b-isoprene). The symbol “b” signifies “block” Although these are di-block copolymers as examples, it will be apparent to the skilled person that self-assembly may also employ tri-block, tetra-block or other multi-block copolymers.
One prior art method used to guide or direct self-assembly of polymers (such as BCPs) onto substrate surfaces is known as graphoepitaxy. This method involves the self-organization of BCPs guided by topological pre-patterning on the substrate using features constructed of resist (or features transferred from resist onto a substrate surface, or features transferred onto film stacks deposited on the substrate surface). The pre-patterning is used to form an enclosure or “recess” comprising a substrate base and side-walls of resist (or side-walls formed in a film or side-walls formed in the substrate).
Typically, the height of features of a graphoepitaxy template is of the order of the thickness of the BCP layer to be ordered, so may be, for instance, from about 20 nm to about 150 nm.
Lamellar self-assembled BCPs can form parallel linear patterns of lithography features with adjacent lines of the different polymer block domains in the recesses. For instance if the BCP is a di-block copolymer with A and B blocks within the polymer chain, the BCP may self-assemble into an ordered layer in each recess, the layer comprising regularly spaced first domains of A blocks, alternating with second domains of B blocks.
Similarly, cylindrical self-assembled BCPs can also form patterns of lithography features comprising cylindrical discontinuous first domains surrounded by a second continuous domain. For instance, if the BCP is a di-block copolymer with A and B blocks within the polymer chain, the A blocks may assemble into a cylindrical discontinuous domain within a circular recess and surrounded by a continuous domain of B blocks. Alternatively, the A blocks may assemble into cylindrical discontinuous domains regularly spaced across a linear recess and surrounded by a continuous domain of B blocks.
Graphoepitaxy may be used, therefore, to guide the self-organization of lamellar or cylindrical phases such that the BCP pattern subdivides the spacing of the side walls of a recess into domains of discrete copolymer patterns.
In a process to implement the use of BCP self-assembly in nanofabrication, a substrate may be modified with a neutral orientation control layer, as part of the graphoepitaxy template, to induce the preferred orientation of the self-assembly pattern in relation to the substrate. For some BCPs used in self-assemblable polymer layers, there may be a preferential interaction between one of the blocks and the substrate surface that may result in orientation. For instance, for a polystyrene (PS)-b-PMMA block copolymer, the PMMA block will preferentially wet (i.e. have a high chemical affinity with) oxide surfaces and this may be used to induce the self-assembled pattern to lie oriented parallel to the plane of the surface. Normal orientation may be induced, for instance, by depositing a neutral orientation layer onto the surface rendering the substrate surface neutral to both blocks, in other words the neutral orientation layer has a similar chemical affinity for each block, such that both blocks wet the neutral orientation layer at the surface in a similar manner. By “normal orientation” it is meant that the domains of each block will be positioned side-by-side at the substrate surface, with the interfacial regions between adjacent domains of different blocks lying substantially perpendicular to the plane of the surface.
In a graphoepitaxy template for aligning a di-block copolymer having A and B blocks, where A is hydrophilic and B is hydrophobic in nature, the graphoepitaxy pattern may comprise hydrophobic resist side-wall features, with a neutral orientation base between the hydrophobic resist features. The B domains may preferentially assemble alongside the hydrophobic resist features, with several alternating domains of A and B blocks aligned over the neutral orientation regions between the pinning resist features of the graphoepitaxy template.
Neutral orientation layers may be created by, for instance, use of random copolymer brushes which are covalently linked to the substrate by reaction of a hydroxyl terminal group, or some other reactive end group, with oxide at the substrate surface. In other arrangements for neutral orientation layer formation, crosslinkable random copolymers or appropriate silanes (i.e. molecules with a substituted reactive silane such as a (tri)chlorosilane or (tri)methoxysilane, also known as silyl, end group) may be used to render surfaces neutral by acting as an intermediate layer between the substrate surface and the layer of self-assemblable polymer. Such silane based neutral orientation layers will typically be present as a monolayer whereas crosslinkable polymers are typically not present as a monolayer, and may have a layer thickness of typically less than about 20 nm.
A thin layer of self-assemblable BCP may be deposited onto a substrate, having a graphoepitaxy template as set out above. A suitable method for deposition of the self-assemblable polymer is spin coating, as this process is capable of providing well defined, uniform, thin layers of self-assemblable polymer. A suitable layer thickness for deposited self-assemblable polymer films is approximately about 10 to about 150 nm.
Following deposition of the BCP film, the film may still be disordered or only partially ordered and additional steps may be needed to promote and/or complete self-assembly. For instance, the self-assemblable polymer may be deposited as a solution in a solvent, with solvent removal, for instance by evaporation, required prior to self-assembly.
Self-assembly of BCPs is a process where the assembly of many small components (the BCPs) results in the formation of larger more complex structures (the nanometer sized features in the self-assembled pattern, referred to as domains in this specification). Defects arise naturally from the physics controlling the self-assembly of the polymers. Self-assembly is driven by the differences in interactions (i.e. differences in mutual chemical affinity) between A/A, B/B and A/B (or B/A) block pairs of an A-B block copolymer, with the driving force for phase separation described by Flory-Huggins theory for the system under consideration. The use of graphoepitaxy may greatly reduce defect formation. The Flory-Huggins interaction parameter (chi value), and the degree of polymerisation of the BCP blocks (N value) are parameters of the BCPs which affect the phase separation, and the dimensions with which self-assembly of a particular BCP occurs.
For polymers which undergo self-assembly, the self-assemblable polymer will exhibit an order-disorder temperature To/d. To/d may be measured by any suitable technique for assessing the ordered/disordered state of the polymer, such as differential scanning calorimetry (DSC). If layer formation takes place below this temperature, the molecules will be driven to self-assemble. Above the temperature To/d, a disordered layer will be formed with the entropy contribution from disordered A/B domains outweighing the enthalpy contribution arising from favourable interactions between neighbouring A-A and B-B block pairs in the layer. The self-assemblable polymer may also exhibit a glass transition temperature Tg below which the polymer is effectively immobilized and above which the copolymer molecules may still reorient within a layer relative to neighbouring copolymer molecules. The glass transition temperature is also suitably measured by DSC.
Defects formed during ordering as set out above may be partly removed by annealing. Defects such as disclinations (which are line defects in which rotational symmetry is violated, e.g. where there is a defect in the orientation of a director) may be annihilated by pairing with other defects or disclinations of opposite sign. Chain mobility of the self-assemblable polymer may be a crucial factor for determining defect migration and annihilation and so annealing may be carried out at temperatures where chain mobility is high but the self-assembled ordered pattern is not lost. This implies temperatures up to a few ° C. below the order/disorder temperature To/d for the polymer.
Ordering and defect annihilation may be combined into a single annealing process or a plurality of processes may be used in order to provide a layer of self-assembled polymer such as BCP, having an ordered pattern of domains of differing chemical types (i.e. of domains of different block types).
In order to transfer a pattern, such as a device architecture or topology, from the self-assembled polymer layer into the substrate upon which the self-assembled polymer is deposited, typically a first domain type will be removed by so-called breakthrough etching to provide a pattern of a second domain type on the surface of the substrate with the substrate laid bare between the lithography features of the second domain type. Patterns having parallel cylindrical phase domains can be etched using dry etching or reactive ion etching techniques. Patterns having lamellar phase domains can utilise wet etching techniques in addition to those suitable for the etching of parallel cylindrical phase domains.
Following the breakthrough etching, the pattern of the ordered BCP may be transferred by so-called transfer etching using an etching means which is resisted by the second domain type and so forms recesses in the substrate surface where the surface has been laid bare.
Spacing between lithography features is known as pitch—defined as the width of one repeat unit of the lithography feature (i.e. feature width plus inter-feature spacing). Self-assembly processes using BCPs can be used to produce lithography features with particularly low pitch, typically less than 30-50 nm.
Self-assembly of BCPs is also controlled by the spacing of photo-resist walls and the BCP material thickness. The thickness of the BCP layer within a graphoepitaxy template may be optimised for the formation of distinct domains of type A and type B polymers within regions of the graphoepitaxy template. The placement of the domains of type A and type B polymers within regions of the graphoepitaxy template may be guided by the arrangement of the graphoepitaxy template.
For example, a circular recess may be defined on a substrate surface. A deposited BCP layer may be caused to self-assemble within the circular recess to form distinct domains of polymers. A first type A polymer domain may be formed as a cylinder within a continuous type B polymer domain within the recess. Breakthrough etching may be used to remove the cylindrical type A polymer domain, resulting in the formation of a circular opening. The circular opening may be centrally located within the circular recess and may allow further processing to be carried out on the substrate, such as, for example, etching of the substrate in the region of the circular opening. It will be appreciated that the placement of the opening with respect to the placement of the recess controls the accuracy of the placement of the further processing carried out on the substrate.
It would be useful to be able to construct multiple BCP features on a substrate with a substantially predictable placement.
It is an object of the invention to obviate or mitigate one or more disadvantage associated with the prior art.